Blender 2.81 doesn't seem to be importing vertex color for .ply
Blender's ply importer only adds vertex colors when they are part of a face or triangle strip. You can see this in the source code of import_ply.py. 

mesh_colors is the list that collects the vertex colors. The assignment of values to mesh_colors and the indices to mesh_faces happens in the add_face() function. This function is only called when a b'face' or b'tristrips' is encountered in the ply object. Only when mesh_faces is not empty and the required color channels are present the new set of vertex colors is created through mesh.vertex_colors.new(). Therefore vertex colors are only imported if the colored vertices were part of a face.